The invention relates to a gyrator circuit comprising a first and a second transconductance amplifier A and B respectively having opposite conductances, which amplifiers are arranged in parallel between a first terminal a and a second terminal b and comprises a first capacitance C.sub.1 arranged between said first terminal a and a third terminal m, which gyrator circuit simulates an inductance L.sub.g arranged between said second terminal b and said third terminal m and comprises means for controlling the quality factor Q of said inductance.
The invention also relates to a resonant circuit including said inductance L.sub.g.
The invention is used for constructing filters for microwave frequencies in the range from 2 to 5 GHz and for realising oscillators.
A gyrator circuit which simulates an inductance and which is included in a resonant filter circuit is known from the publication in "IEEE, J. Solid-State Circuits, Vol. SC-15, pp. 963-968, December 1980", from the article entitled "Gyrator Video Filter IC with Automatic Tuning" by Kenneth W. MOULDING et al. This document describes a gyrator circuit comprising two differential amplifiers operated as voltage-controlled current sources having opposite transconductances, which are arranged in parallel head-to-tail and which are loaded by two capacitances. The technology used for realising this circuit utilizes bipolar transistors having such a threshold voltage that a substantial phase shift is produced in the amplifiers, which results in a negative resistance whose absolute value is higher than the positive resistance caused by losses, so that the circuit is susceptible to oscillate. For this reason this circuit is provided with an additional resistance to raise the positive resistance caused by losses in order to compensate for this fault and to preclude oscillation. Since the quality factor of such a circuit is inversely proportional to the resultant of the resistance due to the phase shift and the resistance due to losses, this additional resistance must be very low. In this known circuit said additional resistance is arranged in series with one of the load capacitances of the gyrator and is connected to a bias voltage by means of which the tuning of the resonator can be adjusted.
However, this known circuit should meet the requirement that it can be tuned to a given low frequency, whilst a satisfactory quality factor is maintained. The problems encountered in realising this circuit stem from the spread in characteristics of the component and in particular the realisation of the resistances.
Therefore, in order to render this circuit independent of the problems of the spread in characteristics, an auxiliary gyrator circuit tuned by means of a quartz oscillator is utilised for tuning the filter gyrator and to provide the bias voltage for the capacitances. This system enables the auxiliary gyrator circuit to be controlled in a closed loop but it is based on the fact that the filter gyrator must keep in track.
For the envisaged use at microwave frequencies this known circuit has several drawbacks:
First of all, the operating frequency is too low (&lt;10 MHz); PA1 Further, the spread in characteristics and, in particular, those of the resistors necessitates the use of an auxiliary gyrator. PA1 Finally, the filter gyrator is merely driven by the auxiliary gyrator but not in a controlled manner. PA1 the operation frequency is high, i.e. is situated in the range between 2 kHz and 4.0 MHz; PA1 the quality factor is high; PA1 the filter frequency is accurate and the circuit characteristics are reproducible. PA1 It is possible to use gallium-arsenide FETs and thus to obtain high frequencies. PA1 The two principal circuit parameters, namely the positive resistance due to losses and the negative resistance due to the phase shift are controlled, in contra-distinction to the known device which only comprises a means for influencing the resistance due to losses. PA1 The quality factor is truly controlled. PA1 The quality factor is now very high of the order of 250. PA1 The circuit is simpler than the known device and can be wholly integrated. PA1 the operating frequency is now very high, PA1 the filter frequency is very accurate. PA1 The characteristics of this circuit are reproducible and consequently suitable for fabrication in large series. PA1 the frequency band which is covered is very wide: 2 to 5 GHz, PA1 the circuit occupies a small area and has a very low power consumption because the transistors and capacitors can be very small.
However, for this envisaged use, it is essential that:
In accordance with the invention this object is achieved by means of a circuit as defined in the opening paragraph, which is characterized in that the first transconductance amplifier circuit A comprises two inverting amplifier stages P.sub.1 and P.sub.2 arranged in series, in that the second transconductance amplifier circuit B comprises an inverting amplifier stage P.sub.3, and in that the means for controlling the quality factor Q comprise a first means P.sub.5 for influencing the output transconductance g.sub.2 of the gyrator and a second means P.sub.6 for influencing the phase shift .phi. between the output current and the gyrator control voltage.
This gyrator circuit has inter alia the following advantages:
For the envisaged use as a resonant circuit to form a filter:
For the envisaged use as an oscillator: